As is common knowledge, a numerical control machine tool includes a mechanical structure with a spindle which carries a tool for machining objects and makes it rotate, and an electronic control unit to precisely control the spindle displacements along three or more axes of movement and the tool rotational speed.
The tool of a machine tool has to be measured, also when it is fast rotating about its axis, to determine its effective dimensions once it is mounted on the spindle or to determine its wear after some working hours. For this purpose, the machine tools are equipped with an automatic measuring system which enables to measure the geometrical features of the tool, including its dimensions, while it is rotating about its own axis.
An automatic measuring system is known, that includes a shadow casting vision system which comprises a source of unfocused light and a two-dimensional image sensor, for instance a CCD sensor, placed in front of, and at a certain distance from, the light source. When used, the tool to be measured must be placed, while it is rotating about its own axis, between the light source and the image sensor, within the visual field of the latter, in such a way that the image sensor can acquire images of the tool shadow. The geometrical features of the tool are measured on the basis of the images acquired by means of the vision system.
In order to carry out the wanted measurements, the vision system has to acquire images of the tool, that is rotating about the rotating axis, at different angular positions which are spaced apart from one another of a certain angular step. The rotating period of the tool to be measured is usually much shorter than the frame acquisition period of the image sensor. Therefore, in order to obtain images of the rotating tool with the wanted angular step, the vision system acquires images according to an acquisition period so that the tool performs, between two consecutive acquisitions, a certain integer number of complete revolutions plus a fraction of revolution equal to a wanted angular step.
In order to actually obtain images in the wanted angular positions, the rotational speed of the tool must be known with high precision. Indeed, it is possible to demonstrate that even differences of 1 part over 10000 between the nominal or known speed and the actual speed can lead to big acquisition errors, i.e. to obtaining images at angular positions that are far away from the wanted angular positions.
Thus far, two methods are essentially known to solve possible deviations of the actual speed value from the nominal one. A first known method consists in performing a number of acquisitions that is hugely redundant compared to a minimum number of acquisitions. This first method is often not feasible since too much execution time is required in comparison with the one allowed for carrying out the measuring cycle. The second known method consists in using a speed or position sensor arranged, for instance, on the spindle, in such a way that the speed real-time data, always updated and reliable, are available. This second method is in many cases not relished for it is considered too much invasive.